The curved glassy structures - or "Fosteritos" - that announce the Bilbao Metro network at street level are as unique to Bilbao as the Art Nouveau Metro entrances are to Paris. Their shape is evocative of inclined movement, generated by the profile of the tunnels themselves. They admit natural light by day, and are illuminated at night, forming welcoming beacons in the streetscape. The curved forms of these spaces are expressive of the enormous forces they are designed to withstand. The lightweight elements - mezzanines, stairs, ticket barriers - are designed to be maintenance-free. All services are confined to plugs at the ends of the station caverns. Ventilation ducts and electrical cables run below the platforms, and trains are powered from an overhead source.

Many subway systems are difficult to negotiate, relying on signage systems. In Bilbao the architecture itself is legible. Routes in and out - via escalators or glass lifts - lead as directly as possible through tunnels to cavernous stations. These are large enough to accommodate mezzanines and staircases above the trains - the experience of moving through a single grand volume is dramatic, while the concept offers flexibility for future change and makes the spaces clearly legible. The Bilbao Metro serves the one million inhabitants of Bilbao.

Bilbao has a strong tradition of technology; most of the elements were made locally and Spanish engineers who had pioneered mobile gantries for the aerospace industry exploited this technology to erect the prefabricated concrete panels lining the Metro's tunnels. A metro system is an excellent demonstration of how the built environment influences the quality of our lives. The building of tunnels for trains is usually seen in isolation from the provision of spaces for people - even though they are part of a continuous experience for the traveller, starting and ending at street level. The Bilbao Metro is unusual in that it was conceived as a totality: architectural, engineering and construction skills were integrated within a shared vision. An addition to the network, Line 2, is extending the system into the Left Bank of the River Nervion. The practice is designing eight of the ten stations on the new line, which is a vital part of the regeneration of the city's former industrial suburbs. Steep hills make it impossible to use escalators to access the new stations, so banks of large-capacity lifts have been designed, grouped in threes to create iconic and easily recognisable entrances.